Install Guide: Small edits on neutron

Add line breaks as needed for screens
Move agent plugin install at the right place for openSUSE, fix description
Fix typo
Use NEUTRON_PASS,  service, controller consistently
Add some FIXMEs to remind us to add openSUSE/Fedora instructions

Change-Id: I3776ff8bf328840c3eae14ee236ebac8243cd959
backport: havana
(cherry picked from commit 0638317285b1819208937a4f01659717f6b4ce6b)
Merge "Install Guide: Small edits on neutron" into stable/havana